In an electrographic process, a dielectric member, such as a photoconductive element, is initially uniformly electrically charged. An electrostatic latent image charge pattern is formed on the dielectric member by image-wise exposing the dielectric member to a suitable exposure source. For example, if the dielectric member is a photoconductive element, the photoconductive element is exposed by an exposure source such as a laser scanner or an LED array. The latent image charge pattern is developed into a visible image by bringing the electrostatic latent image charge pattern into close proximity to a developer material such as contained in a magnetic brush or other known type of development station. The developer material is typically formed of two or more components with non-marking magnetic carrier particles and marking non-magnetic toner particles adhering to the carrier particles. With the latent image charge pattern, on the dielectric member, in close proximity to the developer material, the toner particles are attracted, and adhere to, the dielectric member by the charge pattern. The resulting toner particle developed image is subsequently transferred to a receiver member, such as a paper or a plastic sheet for example, preferably by using an electrostatic field to urge the toner particles in the direction of the receiver member. The electrostatic field is commonly applied in one of several ways. For example, charge can be sprayed on to the back of a receiver member using a corona device. However, it is frequently preferable to use an electrically biased transfer roller to apply the field. Upon completion of the transfer of the toner particle developed image to a receiver member, the developed image is fused to the receiver member by application of heat and/or pressure, for example.
Many mechanisms serve to effect density of an image reproduced in an electrographic engine. When the dielectric member is a photoconductive element, photoconductive element voltages, developer station bias voltages, toner charge, transfer efficiencies from imaging members to receivers, and image fixing can all have an adverse effect on image density. Normally, closely controlling toner charge is attempted to achieve subsystem voltages that are manageable. This control can be accomplished in many ways. Toner concentration has a direct, inverse effect on the toner charge. Dry chemical additives such as silicas, titanias, and stearates also affect the toner charge. Small particle additives such as silicas can be very helpful as transfer release agents, but can also add to the water content sensitivity of toner charge. Toner takeout rates and additive embedment are difficulties that can affect the toner charge in a way that is not controllable. The addition of these additives also adds cost and time to toner production.
One of the larger contributors to the toner charge variability, is the environmental conditions that occur in and around the development station. As water content increases, toner charge decreases. Warmers, driers, humidifiers, and additives have been used to combat or control this, all with an eye to controlling the effect of water on the toner charge. U.S. Patent Application Publication No. 2004/0042815, published on Mar. 4, 2004, in the names of Wayman et al., shows a humidification system for a development station to control charge on toner particles for developing a latent image charge pattern. The humidification is provided by adding water vapor to an airflow directed into the development station. The addition of water vapor is not as precise as would be required to enable an accurate control over the toner particle charge.